The objective of this study is to determine the acute safety of a cold-adapted (ca) temperature-sensitive (ts) parainfluenza virus type 3 (PIV3) vaccine (PIV3-cp45) in healthy adults, PIV3-seropositive children and infants aged 12-60 months, and PIV3-seronegative infants aged 6-36 months. Analysis of safety will include clinical monitoring for signs and symptoms of systemic or respiratory illness, measurements of the duration and degree of vaccine virus shedding, and evaluations to determine the phenotypic stability of any PIV3-cp45 isolates recovered, and the risk of transmission to susceptible contracts of vaccinees. Our adult phase of the study is complete. We continue to recruit infants from our practice.